This invention relates to bonded products, to methods of fabrication therefor and to bonding materials for use in such methods.
There are a number of ways in which bonded products, comprising components that have been bonded together, can be fabricated. For example the components may be bonded by welding, soldering, or by the use of an adhesive. Welding involves melting the components so that they bond together. Alternatively one of the components may comprise a bonding material such as an adhesive or a solder. For example if one of the components is a solder, then this component may be melted to form a bond between the other components. Welding technology has a particularly important role to play in the field of silicon microfabrication, which is now an established manufacturing technique for producing micromechanical devices. The technique provides for batch-processing miniaturised silicon devices of great diversity, for example micropumps, accelerometers, pressure sensors and microactuators. Many micromechanical devices comprise several micromachined components, each component being formed from bulk crystalline silicon. Assembly of such devices often involves joining parts of silicon wafers, comprising bulk crystalline silicon, together in a spatially precise, clean-room compatible manner. Silicon wafer bonding technology is therefore an important aspect of device manufacture, for example to ensure that assembled and packaged devices maintain operational reliability.
Many microfabricated devices incorporate electronic circuits, for example circuits to perform in-situ signal processing or provide drive signals for operating the devices. For many applications, the circuits have to be protected from an environment in which the devices are to be used. Such protection is conventionally achieved by encapsulating the devices in respective packages which are sealed by forming a package hermetic seal under vacuum conditions
There are presently two dominant conventional bonding processes for bonding silicon-based components together, namely xe2x80x9cdirect bondingxe2x80x9d and xe2x80x9canodic bondingxe2x80x9d. In direct bonding two or more components, comprising bulk crystalline silicon, are assembled so that the surfaces to be bonded are in contact with each other. Heat is then applied to the assemble components so that the associated surfaces form a bond. For many applications temperatures approaching 1000xc2x0 C. are required before the bond can be formed. In contrast, anodic bonding is often employed to form bonds between silicon and silica components. It involves mating a polished surface of a silicon component to that of a silica component to be joined together and then applying a high electric field across an interface formed where the surfaces mate, thereby mutually polarising the surfaces to form an electrostatic bond at the interface. During anodic bonding, heating the components enhances bonding strength achievable therebetween.
Both of these conventional bonding processes described above suffer a disadvantage that the components need to be heated in their entirety for direct bonding and high electric field strengths are required for anodic bonding. In many situations, electronic circuits are not capable of withstanding annealing temperatures used in direct bonding and high electric field strengths applied in anodic bonding; aluminium interconnections cannot withstand temperatures in excess of 450xc2x0 C. for example, whereas high electric fields can damage or ionise silicon nitride or silicon dioxide dielectric layers for example. Moreover, bonding strengths provided by direct bonding and anodic bonding are insufficient in certain device applications where high reliability is paramount, for example for micromachined accelerometers which are to be subjected to peak acceleration forces in excess of 25000 g.
A demanding application for encapsulated microfabricated micromachined devices is in biological environments where there are, for example, corrosive biological body fluids. Providing protection from such fluids is particularly important for safety-critical applications where device failure cannot be tolerated, for example in amicrofabricated pace maker arranged to provide heart stimulation. A conventional approach for protecting electronic circuits for use in biological systems is to encapsulate them within welded titanium boxes, titanium being a biocompatible material which biological systems accept by forming a layer of cells thereonto which thereby avoids biological rejection problems. This conventional approach was developed in the 1960""s and 1970""s where, even in that era, hermetic seals were of a sufficiently high quality to realise a remarkably low failure rate; J Buffet in an article in Medical Progress in Technology 1975 Vol. 3 page 133 reported a failure rate of 13 out of 5800 implanted pacemakers encapsulated within welded titanium enclosures over a three year period.
Although adoption of welded titanium enclosures has been acceptable to health care industries generally, the enclosures tend to be bulky which excludes their use in situations where miniaturisation is of prime importance, for example for incorporation into an human inner ear region to stimulate nerve endings therein. In a publication Advanced Materials 7, 1995 pp. 1033, it is disclosed that silicon is potentially usable, instead of titanium, for enclosures for use in biological systems. Bonded silicon microfabricated micromachined components can thereby not only form devices suitable for use in biological environments but also provide their own encapsulation. However, especially in safety critical applications, seals provided between bonded silicon components must be extremely reliable. Conventional bonding techniques, for example direct bonding and anodic bonding, are often insufficiently reliable for safety critical applications. There is therefore a need for a more reliable bonding technique for bonding together semiconductor components.
Silicon welding has been previously investigated during the 1960""s and 1970""s and is reported in an article by H Foll and D G Ast in the proceedings of the Ninth International Conference Electron Microscopy, 1978, pp. 428-429. It was quickly abandoned as a reliable process for bonding silicon components because:
(a) welding of silicon components requires them to be heated to an elevated temperature, namely bulk crystalline silicon has a melting point temperature of 1414xc2x0 C. which means that silicon components to be bonded by conventional silicon welding have to be heated to this temperature; such a high melting point is incompatible with other microcircuit parts, for example aluminium metallisation in an integrated circuit cannot withstand temperatures in excess of 450xc2x0 C.;
(b) silicon is a brittle material and exhibits a high thermal budget for making it fuse during welding; this greatly increases likelihood of fracture from thermally induced stresses.
It has been reported, by Goldstein in Appl. Phys. A62, p 33-7 (1996), that nanocrystals of silicon, comprising porous silicon, melt at lower temperatures than bulk crystalline silicon. Melting temperatures as low as 200xc2x0 C., for 4 nm diameter nanocrystals of silicon have been reported, which compares with melting temperatures of 1414 for bulk crystalline silicon. Porous silicon may be fabricated by the chemical dissolution of bulk crystalline silicon as described by L T Canham in Appl. Phys. Lett. Vol 57, p1046 (1990). Provided the pores are sufficiently closely spaced, nanocrystalline silicon can be formed by this technique.
The following items of prior art are relevant to this invention: U.S. Pat. No. 5,628,848, WO 9606700, EP 0461 481 A2, GB 2337255, and GB 2317885. U.S. Pat. No. 5,628,848 relates to the formation of multilayer structures that are sintered together to form a strong bond between the layers. The starting materials for the layers are in the form of powders. WO 9606700 relates to the fabrication of nanoscale particles. The invention also relates to the use of nanoscale particles to join components together. EP 0461481 A2 relates to the use of nanocrystalline material in welding ceramic components together. GB 2337255 and GB 2317885 relate to the use of silicon for biological and medical application.
It is an objective of the invention to provide new bonded products and methods for fabricating such products that reduce the above mentioned problems. It is a further objective of the invention to provide new bonding materials for use in bonding methods.
According to a first aspect, the invention provides a method of fabricating a bonded product comprising at least two components that are bonded together, the method comprising the steps of:
(a) bringing the components together; and
(b) heating the components;
wherein at least one of the components comprises a nanomaterial and wherein steps (a) and (b) are performed in such a manner that the components are bonded together by heating at least part of the nanomaterial.
Step (b) may be performed prior to, during, or after step (a). Step (a) may comprise the step of abutting each component with the or at least one of the components.
One of the components may comprise all of the nanomaterial. Alternatively each component may comprise nanomaterial.
The method may be used to weld or to solder the components together.
The method provides the benefit that the components can be welded together at a lower bonding temperature compared to conventional bonding techniques. This reduces the chances of fracture from thermally induced stresses.
For the purposes of the invention, a nanomaterial is defined as a material comprising wires or particles having at least one dimension in a range of 1 nm to 20 nm. The nanomaterial may comprise a nanocrystalline material; the nanocrystalline material comprising crystals having a smallest dimension in the range 1 nm to 20 nm.
Preferably the nanomaterial comprises wires or particles having a smallest dimension in a range in which fusion temperatures of nanocrystals of the material are lower than corresponding bulk crystalline material.
Advantageously at least one of the components comprises a semiconducting material; more preferably at least one of the components comprises silicon; yet more preferably at least one of the components comprises bulk crystalline silicon.
Preferably at least one of the components comprises a semiconducting material; more preferably at least one of the components comprises silicon.
Advantageously the nanomaterial comprises a semiconducting material, more preferably the nanomaterial comprises silicon; yet more preferably the silicon comprises porous silicon; even more preferably the porous silicon has a porosity in the range 30% to 90%.
Preferably the method further comprises the step of forming at least one of the components by: (i) taking a sample of silicon, and (ii) anodising at least part of said silicon to form porous silicon.
In this way porous silicon may be formed on the surface of a component to be bonded. The porous silicon is, at least initially, integral with the silicon from which it is formed. The attachment of the porous silicon to a component, or the rest of a component, assists in positioning the nanomaterial for bonding.
Advantageously step (b) comprises the step of passing an electric current through at least part of the nanomaterial; more preferably step (b) comprises the step of passing an electric current through at least part of the nanomaterial for a period between 20 and 80 seconds; yet more preferably step (b) comprises the step of passing an electric current through the nanomaterial and the or at least two of the components.
The passage of an electric current allows heat to be applied at a particular location. Heating occurs preferentially at the nanomaterial, due to its relatively high electric resistance, and not in the surrounding region. The use of two electrodes may further restrict current flow to the region between the electrodes. The use of a semiconducting material, as opposed to an insulator, opens the way for such electrical heating.
Porous silicon exhibits lower thermal conductivity than bulk crystalline silicon. Bulk silicon has a thermal conductivity of 150 W mxe2x88x921Kxe2x88x921, whereas porous silicon exhibits thermal conductivities in a range of 140 W mxe2x88x921Kxe2x88x921 to below 1 W mxe2x88x921Kxe2x88x921 depending upon porosity and associated nanocrystal diameter. Reduced thermal conductivity is beneficial because it allows higher localised temperatures to be achieved in porous silicon using electrically resistive heating.
Porous silicon also exhibits a higher electrical resistivity relative to bulk crystalline silicon, localised heating for reliable bonding is achievable for lower electrical energy inputs compared to that required for bulk silicon components devoid of porous silicon. This provides a benefit that the components do not need to be heated to as high a temperature as would be required for bulk silicon components devoid of the porous material and results in reduced thermal stresses to the bonded product.
Preferably step (b) comprises the step of melting the nanomaterial.
Preferably the method comprises the further step of locating the porous silicon in an inert atmosphere prior to or during step (b). The inert atmosphere may comprise either nitrogen or a nobel gas; the nobel gas may be selected from argon and helium.
Advantageously step (b) is performed at a pressure less than 1 mbar, more advantageously step (b) is performed at pressures less than 10xe2x88x922 mbar, yet more advantageously step (b) is performed at pressures less than 10xe2x88x924 mbar.
Preferably the bonded product is a pharmaceutical product and the method further comprises the step of forming and arranging the components in such a manner that, once bonded, they are suitable for oral consumption by a human or animal.
Advantageously the bonded product is a pharmaceutical product and the method comprises the further step of forming and arranging the components in such a manner that, once bonded, they are suitable for administration to a human or animal in the form of a suppository.
Preferably the bonded product is an implant and the method comprises the further step of forming and arranging the components in such a manner that, once bonded, they are suitable for implantation into an animal or human body; more preferably the bonding is performed in such a manner that, when the components are implanted, a hermetic seal against animal or human body fluids is formed between the bonded components.
Advantageously the method comprises the further step of forming an integrated circuit in one of the components, more advantageously the integrated circuit is a silicon integrated circuit.
Preferably the method further comprises the step of removing any oxygen atoms bonded to the nanomaterial; more preferably the oxygen removing step comprises the step of treating the nanomaterial with hydrofluoric acid.
Advantageously step (b) comprises the step of heating the nanomaterial by radiating the nanomaterial with laser radiation.
At least one of the components may be a micromachined component.
The method is of particular value in the fabrication of bonded products comprising micromachined components. This is because micromachined components have a relatively low mass and are therefore particularly vulnerable to thermal shock, and because the method may be performed at relatively low temperatures.
According to a second aspect, the invention provides a bonded product comprising fused nanomaterial characterised in that at least part of the fused nanomaterial forms a bond between a first part of the bonded product and a second part of the bonded product.
Preferably the bonded product comprises a semiconductor material, more preferably the semiconductor comprises silicon; yet more preferably the silicon comprises bulk crystalline silicon and/or polycrystalline silicon and/or porous silicon.
Advantageously the fused nanomaterial comprises fused nanocrysatiline silicon, more advantageously the fused nanomaterial comprises fused porous silicon.
Preferably the bonded product has a form and composition such that it is suitable for oral consumption by an animal or human body.
Advantageously the bonded product has a form and composition such that it is suitable for inclusion in a suppository.
Preferably the bonded product has a form and composition such that it is suitable for implantation in an animal or human body; more preferably at least part of the fused nanomaterial is arranged such that, when implanted, a hermetic seal against animal or human body fluids is formed between the first and second parts of the bonded product.
The bonded product may further comprise a micromachined component. The bonded product may comprise an integrated circuit.
The method is of particular value in the fabrication of bonded products comprising integrated circuits. This is because integrated circuits have delicate circuitry that is particularly vulnerable to thermal shock, and because the method may be performed at relatively low temperatures.
According to a third aspect, the invention provides a bonding material comprising a nanomaterial.
Preferably the bonding material comprises a semiconducting nanomaterial; more preferably the semiconducting material comprises silicon; yet more preferably the silicon comprises porous silicon; even more preferably the porous silicon comprises comprises crushed porous silicon.
Advantageously the nanomaterial comprises a powder.
Preferably the bonding material further comprises a liquid component, the nanomaterial being distributed through the liquid component; yet more preferably the bonding material is in the form of a paste.